Lensatic Compass Guide

I get lots of questions via email from people wanting to know how to use a lensatic compass, and I think I've covered that pretty well in a series of two posts. They give the basics of how they work, how to shoot an azimuth, how to get a bearing, and so on. After reading those two articles anyone should be able to go out and perform basic land navigation with a map and lensatic compass.

What I haven't covered in much detail is some of the nuances of lensatic compasses, such as the tips and tricks to getting the most out of them in the field. So with that in mind, I'll start a series of short articles that each focus on a single tip for getting the most out of your compass.

This one will deal with night and low light navigation, a topic that could span several lengthy articles on its own. I'll get around to that one of these days.

Most people are aware that lensatic compasses are fitted with small amounts of tritium around the dial. This is a low level radioactive isotope that is used in a multitude of applications where self-powered illumination is necessary. It has replaced radium in recent decades after it was discovered that radium exposure is linked to an increased occurrence of bone cancer.

Tritium is considered safe for external use because it is a weak beta particle emitter - so weak in fact that the radiation it emits cannot penetrate human skin. Have no fear, the science here is sound a proven.

It has a half life of about 12 years. This means that at around the 12 year mark, the tritium in your lensatic compass will have radiated about half the energy that it has to offer. At the 24 year mark it will be anotherhalf of that, and so on. So the tritium will continue emitting "light" for quite a long time before it goes completely dark.

If the tritium in your compass has become dim such that you can't read the compass at night, you can "charge" it up with any flashlight. In the event that you're under strict light discipline (in a military setting, for example) you can simply sup your hands around the compass and the head of the flashlight so that no light escapes. Putting the two in a opaque bag or enclosure is effective as well.

The normal method to transfer angles from your lensatic compass to your map and back is with a protractor and a pencil. From your known location on the map, you take your protractor, line it up to true north, find the angle to your next point, adjust for declination, and then shoot that angle with your compass. At that point you know what direction you need to head to get to your next stop, and if you pair that with an accurate distance you'll be on your way in no time.

The only problem is that this process takes time and tools. You need to write things down, draw lines, perform math, and make sure you do so with a reasonable degree of accuracy. There's a lot of room for error and mistakes, and it takes time, which is sometimes a very precious commodity.

There's a better way, I believe, from both a time and an accuracy standpoint.

Since the ultimate goal is to transfer the angle between the map and compass, why not do it directly? That way you skip the math, the protractor, the lines, and get it all done quicker.

So, take your map and lay it down on a flat surface (i.e. the ground). Take your lensatic compass and open it up all the way so it is one flat line. Lay the compass down on the map so its long side is parallel with the north/south axis of the map. I generally line it up with a north/south gridline.

Next, rotate the entire thing (map and compass) until your compass is telling you that it's pointing north. Now your map's true north is lined up to your local magnetic north, and you've just completely circumvented the whole problem of declination (also removing a potential source of error since declination changes with time).

From here it's simple. All you do is take you compass and line it up from your current point to your desired destination and look at the compass reading. That's the heading that you need to move along to reach your desired end (or way) point. Pick up your map and head out!

In the last article we talked about declination, and how magnetic north and true north will vary depending on location and surrounding features. Next we're going to talk about some of the uses of a lensatic compass, starting with the resection.

Resection with a lensatic compass is a process whereby you can determine your own location from that of two known points in the distance. Typically these features are prominent, such as a hilltop, a man-made structure, or an intersection, but they can be more nebulous if you're in a tight spot.

The key to the whole operation is having a map and being able to locate these features on that map. Then, from where you are, shoot an azimuth to the feature as accurately as possible, and calculate the back azimuth from that reading. The back azimuth is nothing more than the opposite direction of the reading you're taking. That is, take the azimuth you recorded and subtract 180 degrees from it, or 1600 mills if using a military lensatic compass.

Next, do the same for the second point.

Now, take your map and draw lines (at the back azimuth angle you calculated) on the map through the features that you're using as reference points. Those lines will intersect at some point if you're done your resection properly. The point where those lines cross is your location, and the accuracy of that location is only as high as the readings and calculations that you've made.

To check your process, and even add a little more accuracy to it, find another feature and perform the same steps. That line should intersect reasonable close to where the other lines crossed. The more back azimuths you can draw, the more accurate your location will be.